The present invention relates to semiconductor fabrication, and in particular to a method for fabricating a pad redistribution layer.
In the fabrication of IC devices, semiconductor chips are frequently attached to other chips or other electronic structures such as a printed circuit board. Attachment of the chip can be accomplished by a wire bonding process or by a flip-chip method. In a wire bonding process, each of a series of I/O bump terminals on a chip built on an aluminum bond pad is sequentially bonded to the connecting pads on a substrate. In a flip-chip attachment method, all the I/O bumps on a semiconductor chip terminate with solder material. In the bonding process, a semiconductor chip is flipped over with the solder bumps aligned and placed in a reflow furnace to effect all the I/O connections to bonding pads on a substrate.
A major processing advantage made possible by flip chip bonding is its applicability to very high density I/O connections and high reliability of interconnects formed, compared to wire bonding. Moreover, wire bonding also presents limitations in the total number of I/O interconnections that can be made in high performance devices.
A limiting factor of flip-chip bonding is the fine pitch of bonding pads that are frequently required with modern high density devices. For instance, in a high density memory device, bonding pads arranged along the periphery of the device may have a pitch, or spacing, as small as 100 μm, making it is difficult and costly to bond to the pads using solder bumps in flip chip bonding, since solder bumps in this case are low profile, making underfill extremely difficult. Moreover, a costly high density substrate is required for flip-chip bonding of devices with fine pitch I/O.
In order to bond high density IC devices having fine pitch peripheral I/O bonding pads, in the range of approximately 100 μm, I/O redistribution must first be carried out before the formation of the solder bumps. In the process, peripheral I/O bonding pads are redistributed by signal traces to area array I/O bonding pads to increase pitch in the area array I/O bonding pads. The I/O redistribution process used in modern high density IC devices is therefore an important fabrication step, to form traces between the varied pairs of bonding pads.
In U.S. Pat. No. 6,511,901, Lam et al. disclose a metal redistribution layer having solderable pads and wire bondable pads, and fabrication thereof. Massive etching is needed to pattern a trimetal layer and form openings therein, forming the metal redistribution layer with increased manufacturing difficulty while forming a copper-comprising redistribution layer. Reliability and topography thereof may thus be affected.